In recent years, rapid progress has been made in development of an organic electroluminescent (organic EL) element, which is a self-emitting element using electrons and holes as carriers. The organic EL element is characterized by a possibility of reducing thickness and weight, good visibility, and the like, compared to those of a liquid crystal element that is a non-self-emitting element which requires a back light.
The organic EL element is generally provided with: a pair of substrates each having an electrode formed on the respective faces opposing to each other; and a light-emitting layer disposed between the pair of substrates. The light-emitting layer is composed of an organic thin film containing a light-emitting substance that emits light when voltage is applied. In the case where such an organic EL is made to emit light, holes and electrons are injected from an anode and a cathode by applying voltage to the organic thin film. Thereby, the holes and electrons are recombined in the organic thin film, and light emission can be obtained when excitons generated by the recombination return to the ground state.
In the organic EL element, a hole injection layer and an electron injection layer for improving efficiency of injecting holes and electrons, and a hole transport layer and an electron transport layer for improving efficiency of recombination of holes and electrons need to be provided between the light-emitting layer and the electrode in addition to the light-emitting layer. By providing such layers, the organic EL element has a multilayer structure, which makes the structure complicated and increases production process. Moreover, the organic EL has a lot of restrictions because a working function needs to be taken into consideration in selecting electrode materials for use in the anode and the cathode.
As a self-emitting element to deal with these problems, light-emitting electrochemical cells (LECs) have been attracting attention in recent years. A light-emitting electrochemical cell generally has a light-emitting layer containing a salt and a light-emitting organic substance. When voltage is applied, cations and anions derived from the salt move toward a cathode and an anode respectively in the light-emitting layer, which brings about a large electric field gradient (electric double layer) in the electrode interfaces. The electric double layer to be formed makes the injection of electrons and holes in the cathode and the anode easy, and therefore the light-emitting electrochemical cell does not need a multilayer structure which is necessary for organic EL elements. Moreover, the light-emitting electrochemical cell does not need to take into consideration a working function of a material to be used as a cathode or an anode and therefore has less restrictions for the materials. From these reasons, the light-emitting electrochemical cell has been expected as a self-emitting element that can reduce production costs greatly when compared with organic EL elements.
Besides a lithium salt and a potassium salt, attempts to use an ionic liquid as the salt for use in light-emitting electrochemical cells have been made. The ionic liquid is a nonvolatile salt and has a high rate of re-orientation by an electric field than solid electrolytes, and therefore mobility of ions is secured to make the formation of an electric double layer easy and make the injection of holes and electrons further easy (refer to Patent Literature 1 and Patent Literature 2).
In the organic EL elements, attempts to make the transfer of holes and electrons easy by adding an additive to a layer which takes on a role of injecting or transporting holes or electrons, such as a hole injection layer, a hole transport layer, an electron injection layer, or an electron transport layer have been made (refer to, for example, Patent Literatures 3 to 6). Moreover, attempts to avoid reduction in light emission luminance by adjusting the balance of injecting holes and electrons to a light-emitting layer have also been made (refer to Patent Literature 7).
On the other hand, the transfer of holes and electrons is already easy in light-emitting electrochemical cells, particularly in light-emitting electrochemical cells having a light-emitting layer containing an ionic liquid; however, attempts to improve the performance of light-emitting electrochemical cells by improving thermal stability and electrochemical stability that give an influence on operating life or by improving adaptability of an ionic liquid with a light-emitting substance have been made. For example, it is described in Patent Literature 8 that the life and the like of light-emitting electrochemical cells are improved when a plurality of salts are incorporated in the light-emitting polymer layer. However, the maximum light emission luminance shown in the literature is merely 100 cd/m2 or lower even when a voltage of 10 V or higher is applied to the electrodes, and therefore a high luminance required in practical use has not been achieved.